Accelerating and maintaining to above 2000 engine rpm is ok in each gear. Deceleration in each gear is fine until the engine rpm gets below 2000 rpm where it starts bucking till you push in the clutch or accelerate. The longer you let it happen the worse it gets. Some engine details : 292 cu in 9.5 comp Cam 242°@050 int & exh ; .500 lift (installed 4° advanced) Ign timing 16° initial @ 1000 rpm; 36 total @ 2200 rpm
Like this idea. Also check for any binding in the clutch linkage. Look for cracks in the z-arm or excess play in the pivot balls.
Carb not closing completely or some reason for allowing excess fuel in during closed throttle operation .
Possibly the diaphragm in the vacuum advance is leaking, causing timing to get wonky at higher engine vacuum...?
Had this vehicle run OK previously? Or is this a new engine, trans, chassis combination? Stock fuel tank or replacement tank with different or no baffling? Leaking carb float or wrong float level? Debris in needle and seat? Check for chafed, stressed or stretched ignition wiring between engine and chassis or body. Diagnosing problems like this can go a lot of different directions. Any more details of your build will be helpful.
This would be hard to solve with the information given. On decel the engine isn't under load so a misfire or lack of fuel isnt likely. Is the exhaust getting stirred up and restricted. Is it backfiring when bucking, caused by electrical short ? A vacuum gauge taped on the windshield might eliminate a Easter egg hunt.
More info: I forgot to say engine is SBF, 289 +.030 Dist is Accel dual point, 39000 series, no vacuum advance, 16° initial, 36° total at 2300. Big yellow coil. Carb is Holley 1860, 600cfm vac secondary with 3310 (780) metering blocks and center hung bowls, front power valve 5.0 rear plugged. Front jets 68, rear 75. Dialed in yeeaars ago, via Holley book and D. Vizard. No issues that I can find. Carb and Dist and Tach been happy together for over 25 years. Bucking gets quite severe, the longer you let it happen the worse it gets. Great input fellas! I did some checking yesterday and did a few things per above posts: Checked engine mounts, clutch is hyd so no binding of linkage. All wires look good. No back firing, plugs look really good, clean. Tank is spotless, no difference full or near empty. I had been thinking for a while that it was either electrical or carb so motivated by suggestions I put a clamp on the vac secondary rod to eliminate actuation and disconnected the old Mallory [limiter and shift light] Tach and installed a Vacuum gage. Was screwing plugs back in and it Rained big time! I will test tomorrow.
Put a glass pack on an old 223. Sounded great but decelerating down hill it seemed to backfire a lot. Very loud. About the same time I replaced the points and condenser. The loud piping out the exhaust stopped. Dang it think I also built the carb
This might seem silly but where do you feel the bucking? Could you have a broken leaf spring and your getting some wheel hop like you’d get with spring wrap?
I have to find the 12v power supply for the tach to disconnect, my wiring diagram says there is a 3amp fuse velcroed to bottom of dash,. How hard can that be to find?
Maybe pass transfer slot at idle, "butterfly" , because Not a stock Cam ? Cigarette lighter , most old cars had one 12v power supply, Just for test purposes Tac
Just check the play in the pinion and ring gear, I know it sounds ridiculous but I have seen a similar situation along time ago. It takes no time to just jack up the car and check. Good luck. Pat
So being a manual, do 2 tests before you "jump to conclusions" 1: when you decelerate, switch off the ignition so the car is loading up the engine. [if the shudder stays it isn't timing or carb etc] 2: point the nose of the car downhill and try and reverse up. It could be weak springs in the clutch plate causing a violent shudder on overrun Reversing uphill sometimes accentuates this. Also it could be an oil leak onto the clutch [check that] Put it in gear and get somebody to rock it back and forth while you crawl under and have a good look
""1: when you decelerate, switch off the ignition so the car is loading up the engine. [if the shudder stays it isn't timing or carb etc]"" ""I see that you said it was working for 25 years, did you change anything? I would look at the carb."" A voltage doubler for an 24v accessory from WW11 airplane! Refresher: Found fuse; I lied to myself (how? dunno), no Velcro was residing on bottom lip of dash. Okay; ready for test run if I can sidestep a few chores and the weather holds up. I really wonder at what is actually the problem. Maybe tomorrow..
Yesterday I took car for test ride. All the "Nots" Carb vac secondary. Tach. Voltage Doubler. Rear gear/axle play, about 1/8th at od of drive shaft, about normal. Does the cam at 242°@ .050 have too much duration to run a PCV [Ford valve typical 70'] system?
Do PCV valves go bad? I have seen valves all sludged up from lack of oil changes and clogged but under proper maintance/oil changes etc can they go bad? My evac: https://www.jalopyjournal.com/forum...ir-oil-separator.1337359/page-2#post-15702685
Without knowing the LSA and the ICL numbers on the cam, 242 @ .050 seems kinda wild for a 292. It might be the driving technique that needs attention. What gear ratio are you running?
The only time I can remember engines hiccuping during deceleration is when the ignition timing was set too far advanced. I've had loose brake shoes that were intermittently catching and caused a similar issue on deceleration.
LSA Is probably around 107° [guess] ICL no clue from back then but I think it was put in 4° advanced per instruction for more power on bottom end. Can't find the cam card but pretty sure it is a Crane H310. 1976 edition. Gear 4.11 with 6spd Richmond. Comfortable idle @ 950-1000 rpm. [292=289 +.030] . About the only thing I found wrong is front seal is starting to weep. AND: YOU ARE RIGHT that in hindsight my driving technique had fallen from memory due to brain fade and driving the Explorer too much [got lackadaisical I think] with a dog that makes more noise than the exhaust in the car I been complaining about. After another day, Friday, of not finding anything wrong I thought maybe its me? I fixed a lot of things that were from age, MC, wheel cyl, ect plus regular PM so I hadn't driven it for like 9 months. It came back last night driving around and keeping it above 2k, thats what I had been doing for years, think me pretty much embarrassed.. Thanks to all who posted.
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